1. Field of the Invention
The present invention relates to a heating and cooling system for heating an environmental or process load by removing thermal energy from the earth and/or ambient atmosphere and transferring that energy to the load and, similarly, cooling the load by removing thermal energy therefrom and transferring that energy to the earth, and/or ambient atmosphere for dissipation therein.
2. Description of the Related Art
With the steadily increasing costs of fossil and other depletable types of fuels, which are presently being used to obtain desirable temperature levels in environmental and process loads, greater emphasis is being directed toward developing systems and methods to extract energy either from the vast, virtually unlimited thermal energy stored in the earth or from the ambient atmosphere and transferring that energy to the loads for heating purposes and, reversely, extracting thermal energy from the loads and transferring that energy either to the earth or to the ambient atmosphere for dissipation therein for cooling purposes. One type of previous system for accomplishing such objectives is commonly referred to as a heat pump.
Conventional air-source reverse-cycle heat pump systems are commonly used for providing heating and/or cooling to building environmental spaces, manufacturing processes, and a variety of other usages. Properly used, such systems can be quite effective in environments where the ambient temperature is not extreme. Although generally acceptable performance is obtained in such moderate ambient temperature conditions, such systems leave a lot to be desired during extreme fluctuations in ambient temperatures, wherein substantial reductions in heating and cooling capabilities and in operating efficiencies are realized.
Modifications in air-source heat-pump systems have been attempted to enhance performance, such as incorporating additional booster heat exchangers and/or secondary refrigerant loops. Unfortunately, such modifications have provided only minor enhancements at best.
In recent years, heat-pump systems have been developed which use subterranean heat exchangers whereby the earth is utilized as a heat source and/or sink, as appropriate. Heat-pump systems utilizing the more moderate temperature range of the earth provides efficiencies which are substantially improved over those obtained from air-source heat pump systems. Such earth exchange systems are based on the concept that useful thermal energy could be transferred to and from the earth by the use of subterranean tubes in flow communication with various above ground components.
A refrigerant coolant pumped through such tubes by a compressor serves as a carrier to convey thermal energy absorbed from the earth, as a heat source, to the above ground components for further distribution as desired for heating purposes. Similarly, the coolant carries thermal energy from the above ground components through the subterranean tubes for dissipation of heat energy into the earth, as a heat sink, for cooling purposes.
Unfortunately, a major complication may arise when refrigerant is pumped through the subterranean tubes. First, lubricant oil which characteristically escapes from the compressor while the system is operating is carried along with the refrigerant throughout the system. Due to the lower elevation of the subterranean tubes, the lubricant oil tends to accumulate in the tubes. As a result, the accumulation of the lubricant oil in the subterranean tubes gradually floods those tubes, substantially reducing the ability of the subterranean tubes to perform their originally intended function. Further, the compressor may be gradually deprived of essential lubricant oil, which jeopardizes the continued successful operation of the compressor. As a result, various complicated refrigerant distribution configurations have been utilized in an attempt to control the flow of the refrigerant.
Second, when an energy demand cycle was completed, the system would shut down while waiting for a subsequent demand for energy transfer. As a result, a certain amount of liquid refrigerant then passing through the subterranean tubes would lose its momentum and remain in the subterranean tubes. When the subsequent energy demand occurred, the compressor, which was generally designed for pumping gas as opposed to pumping liquid, would quickly deplete the gaseous refrigerant trapped between the liquid refrigerant in the subterranean tubes and the compressor such that a low pressure condition was quickly created at the inlet of the compressor. Most compressors are designed to interpret such a low pressure condition at the inlet as an indication that insufficient refrigerant exists in the system to function properly. As a result, the compressor would automatically shut down when such a low pressure condition was sensed in order to protect against potential burn-out of the compressor from absence of sufficient refrigerant.
A similar but more pronounced low pressure problem was encountered when a reversible system switched from a heating mode to a cooling mode. This problem arose from an imbalance in the refrigerant capacity which is inherent in a reversing system. The imbalance results from the much larger volume capacity of the subterranean heat exchanger as compared to the volume capacity of the dynamic load heat exchanger. When the operating cycle reversed, additional time was required to transfer the excess refrigerant whereby the refrigerant could assume its appropriate redistribution throughout the system in order to properly function in the reverse mode.
During that transfer time period, the previously described low pressure condition was created at the inlet of the compressor. The generally, relatively short time interval allowed for the low pressure condition at the compressor inlet before automatic shutdown was generally insufficient for the compressor to overcome the inertial resistance of the static refrigerant in the subterranean tubes and to redistribute the refrigerant for the reverse mode. Again, the low pressure condition at the compressor inlet generally caused the compressor to automatically shut down prematurely. Such imbalance was particularly troublesome during a system start-up at the end of an extended heating cycle where the temperature of the earth surrounding the subterranean tubes has been reduced as a result of extraction of thermal energy therefrom. As a result, a large volume of refrigerant could accumulate in the tubes of the subterranean heat exchanger.
A third problem, which was generally observed for prior art heat pumps, was the absence of a mechanism for achieving refrigerant pressure equalization subsequent to system shutdown for reducing start-up loads. Because of the absence of such pressure equalization, the service life of the compressor was reduced.
Previous attempts to circumvent some of the aforesaid problems generally followed either of two approaches: (i) using a plurality of closed loop systems working in combination, with one of such loops horizontally or vertically disposed subterraneously, or (ii) using a vertically disposed, single-closed loop, subterranean exchanger.
The plural loop approach generally utilized indirect heat exchange rather than direct heat exchange. That approach basically employed two or more distinct and separate, cooperating, closed loop systems. A first one of such closed loop systems was sequentially routed through the earth and through an interim heat exchanger transferring thermal energy therebetween. The other one of such closed loop systems, which was sequentially routed through the interim heat exchanger and through a dynamic load heat exchanger for further distribution as desired via techniques commonly known in the heating and cooling industry, operably interacted with the first such closed loop system in the interim exchanger. Thus, through the cooperative effort of the two separate closed loop systems, thermal energy was indirectly transferred between the earth and the environmental or process load.
By using a plural loop approach, the oil deprivation problem was partially resolved by eliminating transmission of the refrigerant and oil through the subterranean tubes, thereby minimizing the quantity of oil which could be drained away from the compressor or by using a non-phase-change heat transfer fluid in the subterranean portion of such a plural loop system. Such double closed loop systems were substantially more complicated, due to the greater number of components required, and were considerably less efficient than properly designed, single closed loop systems.
The liquid-based earth-source systems provided improvements over air-source systems by using the earth's thermal mass as a heat source/sink and by eliminating the ambient air as a heat source/sink. On the negative side, the utilization of secondary heat exchange loops for these systems increases the complexity of these systems by requiring the use of extra pumps, extra heat exchangers, heat transfer fluids in addition to a refrigerant, etc. Though more efficient than air-source systems, this added complexity limited the efficiency obtainable from such systems.
The vertical single-closed-loop approach generally utilized downwardly or vertically inclined subterranean tubes. Such a system could generally be designed to operate in either a heating mode or a cooling mode. Unfortunately, however, the same system generally would not properly function when operated in a reverse mode due to the difference in specific density of gaseous refrigerant relative to that of liquid refrigerant. Specifically, while the transition from liquid to gas could be designed to occur while the refrigerant was passing downwardly in one mode of operation, such transition could occur while the refrigerant was passing upwardly in the reverse mode of operation. Another shortcoming of a prior art vertical loop system was the entrapment of oil at the lower extremities of the vertically oriented tube, thereby depriving the compressor of essential lubricant oil.
Solutions to many of the foregoing problems were taught in U.S. Pat. No. 5,025,634, HEATING AND COOLING APPARATUS, issued Jun. 25, 1991 to William E. Dressier, by eliminating the need for a secondary liquid-based subterranean heat exchanger and replacing it with a refrigerant-based subterranean heat exchanger, with consequent improvement in heat-pump performance and reduction in equipment and maintenance costs.
A disadvantage arising from prolonged usage of the earth-source heat-pump systems still remained: stressing of the earth's ability to transfer and/or store large quantities of thermal energy in the vicinity of the heat exchanger for extended periods of time. This situation was generally particularly noticeable for systems used for manufacturing processes or under-sized environmental space conditioning applications.
An attempted solution to the stressing problem included the augmentation of a liquid-source heat pump with a liquid-heat exchanger loop which integrated both a liquid-based subterranean heat exchanger and a liquid-based fan coil in an attempt to boost the performance of the liquid-source heat pump, such as that taught by Margen in U.S. Pat. No. 4,091,636. In that system, only one or the other of the heat exchangers were operated at any one time. Unfortunately, such integrated systems generally failed to realize optimum operational efficiencies. Further, the integrated refrigerant and liquid subsystems produced a system with substantially increased complexity and maintenance requirements.
In another approach, such as that taught by Gazes et al. in U.S. Pat. No. 4,920,757, a third fan coil was integrated with a refrigerant-based subterranean heat pump design. That system, however, did not employ the additional fan coil as an alternative energy source. Instead, it merely used the coil to control excess refrigerant build-up in the subterranean heat exchanger; during one cycle, it worked serially with the indoor-heat exchanger and, during the other cycle, it worked serially with the subterranean heat exchanger.
In yet another approach, as taught by Tressler in U.S. Pat. No. 5,239,838, two separate heat exchange sources were incorporated into a single heat pump system. That system could either be operated as an air-source heat pump or as a liquid-source heat pump attached to a thermal storage tank, which was in turn heated by a water heater or solar panel. The system was designed to perform as an air-source heat pump or, during the heating cycle, to draw thermal energy from the storage tank. Again, that system did not realize optimum operational efficiencies because it did not coordinate concurrent utilization of both energy sources.
In still another approach, as taught by Blackshaw et al. in U.S. Pat. No. 4,646,538, a system incorporated three heat exchangers: a secondary liquid-based subterranean heat exchanger, an indoor fan coil heat exchanger, and a hot-liquid heat exchanger. Although the system could transfer heat between any two of the heat exchangers, the Blackshaw et al. system did not utilize the third heat exchanger to augment the performance of the subterranean heat exchange loop and optimum efficiencies were not fully realized.
What is needed is a combined heat pump system which realizes optimal operational heating and cooling efficiencies by coordinating concurrent utilization of an earth exchanger in conjunction with an ambient-air exchanger.